1. Field of the Invention
The present invention relates to a continuous process for preparing a pitch with a high softening point, and is particularly directed to a suitable process for preparing a spinning pitch used for the production of carbon fibers, and also relates to a process for efficiently preparing a homogeneous mesophase pitch with a low softening point, which is suitable for producing pitch-based high-performance carbon fibers.
As stated above, main object of the present invention is to provide a continuous process for the production of mesophase pitches for manufacturing high-performance carbon fibers, but the present invention is not limited thereto. For example, the hydrogenated pitch suitably used to achieve the main object can also provide excellent mesophae pitches in batchwise operations, and further, the apparatus for heat treatment of pitch used in the present invention is not only useful for the production of mesophase pitches but also useful for heat treatment of any type of pitches. When taking into account of circumstances above, one of the embodiments of the present invention can be summarized as follows:
That is, the first embodiment relates to a continuous process for efficiently preparing a pitch with a high softening point by dispersing a heavy oil or pitch in a gas stream of an inert gas or superheated vapor as fine oil droplets at 350.degree.-500.degree. C. under a reduced or normal pressure, and bringing the dispersed fine oil droplets into contact with an inert gas or superheated vapor, thereby effecting elimination of light fractions and a proper degree of thermal polymerization. The second embodiment of the present invention can be summarized as follows:
That is, the second embodiment relates to a process for preparing a mesophase pitch for the production of high-performance carbon fibers which comprises: using, as a raw material, a heavy oil or pitch of coal or petroleum origin which is substantially free from a material insoluble in a monocyclic aromatic hydrocarbon solvent; and subjecting said raw material to a successive four-step treatment comprising a first step of heat-treating said raw material in a tubular heater under a specific condition, thus newly producing a component insoluble in a monocyclic aromatic hydrocarbon solvent without producing a quinoline-insoluble component, a second step of distilling or flashing said heat-treated material obtained in the first step to remove a portion of light fractions thus obtaining a thermal-cracked heavy component having specific properties, a third step of recovering from this thermal-cracked heavy component, the component insoluble in a monocyclic aromatic hydrocarbon solvent or other solvent having the dissolving ability equivalent to the monocyclic aromatic hydrocarbon solvent as a high-molecular-weight bituminous material, and a fourth step of obtaining a soluble component by distilling off the solvent from the mother liquor separated in the third step; while recycling whole or a portion of said soluble component produced in the fourth step to the first step; hydrogenating said high-molecular-weight bituminous material obtained in the third step by heat-treating the same in the presence of a hydrogen-donating solvent, thereby obtaining a hydro-treated liquid, or further removing the solvent to obtain a hydrogenated pitch; and heat-treating the hydro-treated liquid or hydrogenated pitch by dispersing it as fine oil droplets under a specific condition to obtain a mesophase pitch.
The third embodiment of the present invention can be drawn up by omitting the requirement of the fourth step, and omitting also the requirement for recirculation of the soluble component to the first step stipulated in the second embodiment just mentioned above.
The fourth embodiment of the present invention can be drawn up by omitting the requirement for conducting the final heat treatment by dispersion to form fine oil droplets stipulated in the second embodiment mentioned before.
All of these embodiments are, of course, within the scope of the present invention.
Pitches with a high softening point are used as a binder for preparing carbon products or the like. The pitch with a high softening point prepared by the process according to the present invention is particularly suitable for use as a raw material for preparing carbon fibers, since light fractions have efficiently been eliminated from the pitch.
According to the process of the present invention, a homogeneous mesophase pitch with a low softening point can be prepared efficiently and constantly.
Carbon fibers are classified into PAN-based carbon fibers prepared from polyacrylonitrile (PAN) and pitch-based carbon fibers prepared from pitches with a high softening point. The pitch-based carbon fibers are further grouped into general-purpose carbon fibers (GP carbon fibers) with a lower strength and modulus of elasticity and are used as high temperature insulating materials or the like, and high-performance carbon fibers (HP carbon fibers) with a higher strength and modulus of elasticity and are used as structural materials for aircraft, industrial robots, sporting goods, and the like. The characteristics of spinning pitches used for preparing these two pitch-based carbon fibers, GP and HP carbon fibers, are quite different. The spinning pitches used for the GP carbon fibers are the so-called isotropic pitches, which exhibit complete isotropy when observed by a polarizing microscope. The spinning pitches used for the HP carbon fibers are the so-called mesophase pitches which contain as major components mesophases, exhibiting optical anisotropy. These two types of pitches are not only quite different texturally from each other when observed by a microscope, but also largely differ in softening points and in solvent-insoluble contents. There are certain characteristics, however, which these two types of pitches must possess in common. Such characteristics include the absence of light fractions which vaporize at a spinning temperature and cause bubbles to form in the pitch, and the absence of solid components or excessively highly polymerized compounds which do not homogeneously melt at the spinning temperature. Generally, the preparation of spinning pitches for preparing HP carbon fibers, i.e., mesophase pitches, requires a more sophisticated technology than does the preparation of spinning pitches for preparing GP carbon fibers. This is due to the higher softening point of the mesophase pitches, requiring a higher spinning temperature, where the presence of a small amount of light fractions greatly affects the characteristics of the product carbon fibers in an adverse way. Another problem is that the mesophase pitches require heat treatment in the preparation process for converting the pitch texture into the mesophase. This heat treatment tends to produce solid materials or excessively polymerized compounds which do not melt at the spinning temperature. This also causes the characteristics of the produced carbon fibers to be greatly impaired. Thus, the production of a spinning pitch for the HP carbon fibers requires more sophisticated technology than the production of spinning pitches for preparing GP carbon fibers.
The process according to the present invention can be applicable for preparing either of the spinning pitches for GP and HP carbon fiber production. However, the process is especially suitable for the preparation of spinning pitches for producing HP carbon fibers.
2. Description of the Prior Art
Hitherto, a major source of the high-performance carbon fiber has been PAN-based carbon fibers which are produced by spinning PAN, rendering them infusible in an oxydizing atmosphere, and carbonizing or graphitizing them in an inert gas atmosphere. In recent years, however, processes were found to produce from pitches high-performance carbon fibers which are competitive or even superior to the PAN-based carbon fibers in their properties. Since pitches are an inexpensive raw material, the findings have drawn a great attention as a route for preparing high-performance carbon fibers at a low cost.
Production of pitches from heavy oils by processes including distillation, heat treatment, hydrogenation, and the like, are known from early in the art. Heavy oils used include coal tar, those by-produced in the cracking of naphtha (naphtha tar), in the cracking of gas oil (pyrolysis tar) or in the catalytic cracking processes (decant oil), liquefied coals, or topping or vacuum resides. The pitches produced by these processes are widely used for the preparation of carbon products.
In preparing the high-performance carbon fibers from a pitch, the spinning pitch must be a so-called mesophase pitch which contains, as a major component, the substance exhibiting an optically anisotropic mesophase when examined on a polarizing microscope.
This mesophase is a kind of liquid crystals which is formed when a heavy oil or a pitch is heat-treated, and its optically anisotropic character is due to an agglomerated layered structure of thermally polymerized planar aromatic molecules. When such mesophase is subjected to melt spinning, the planar aromatic molecules are aligned to the direction of the fiber axis due to the stress exerted to the melt as it passes through a nozzle hole, and this oriented structure can be kept without being disrupted throughout subsequent steps to render it infusible and carbonization steps, and therefore, high-performance carbon fibers having good orientation can be obtained. On the contrary, when an isotropic pitch containing no mesophase is used, such orientation does not occur sufficiently by the stress when molten pitch passes through a nozzle hole because of the insufficient development of planar structure of molecules, and this renders the fibers poorly oriented and produces a carbon fiber with a lower strength, even if it is rendered infusible and carbonized. Therefore, a number of known processes for the manufacture of a high-performance carbon fiber from pitches are directed to the process for preparing mesophase pitches spinnable into the fiber.
In the decade of 1965-1974, the mesophase was considered as equivalent of the substance insoluble in polar solvents such as quinoline and pyridine because of the fact that the mesophase produced by the heat treatment was insoluble in such polar solvents. Subsequent studies on the mesophase, however, have unveiled the fact that the portion of the pitch which exhibits anisotropy under a polarizing microscope is not necessarily the same substances with polar solvent insoluble substances, and further that the mesophase is composed of both polar solvent soluble and insoluble components. It is thus common nowadays to define the term "mesophase" as "a portion exhibiting optical anisotropy when examined on a polarizing microscope". Furthermore, it is general to express the mesophase content by the ratio of areas exhibiting optical anisotropy and isotropy when a pitch is examined on a polarizing microscope.
The mesophase content as determined according to this definition represents a property of a pitch having a great significance on its spinnability as well as the characteristics of the carbon fiber made therefrom. Japanese Patent Laid-open No. Sho 54(1979)-55625 describes a pitch containing essentially 100% of mesophase, and states that it is desirable to reduce an isotropic portion as much as possible, because the presence of isotropic portion interferes with the spinning operation. The reason is that a pitch with a smaller mesophase content tends to separate into two phases in a molten state due to the lower viscosity of the isotropic portion than the anisotropic mesophase. When one tries, however, to increase the mesophase content of a pitch, the softening point and the viscosity become significantly high, making it difficult to spin the pitch. Thus, the most important problem in a process for preparing a high-performance carbon fiber from a mesophase pitch resides in the fact that a significantly high temperature is necessary to use at the spinning stage because of the high softening point of the pitch. Spinning at a temperature of above 350.degree. C. involves such problems as cutting off of fibers and decrease of the fiber strength resulting from decomposition, deterioration, or thermal polymerization of the pitch in the spinning facility. Since a temperature which is 20.degree.-40.degree. C. higher than the Mettler method softening point of the pitch is generally required for the spinning, the softening point of the mesophase pitch must be below 320.degree. C. in order to keep the spinning temperature lower than 350.degree. C. The process described in Japanese Patent Laid-open No. Sho 54(1979)-55625 is a process for heat-treating a pitch at a relatively low temperature for a long period of time, and as described in the specification, the pitch obtained has a considerably high softening point of 330.degree.-350.degree. C., and therefore, spinning is carried out at a high temperature of above 350.degree. C.
Japanese Patent Laid-open No. Sho 58(1983)-154792 discloses a quinoline-soluble mesophase, and states that the content of the quinoline-soluble mesophase in a pitch must be higher than a specific amount because the quinoline or pyridine insoluble mesophase raises the softening point of a mesophase pitch. There is no detailed description in this laid-opened publication about the differences between the quinoline-insoluble and soluble mesophase, but it may easily be understood that a highly polymerized substance with an extraordinarily high molecular weight would be insoluble in quinoline, and therefore, in other words, an attempt for preparing a pitch with a high quinoline-soluble content would lead to an effort to reduce the content of such extra-ordinarily high molecular weight components and to prepare a homogeneous pitch having a narrow molecular weight distribution. The process of Japanese Patent Laid-open No. Sho 58(1983)-154792 comprises heat-treating a pitch having a specific range of aromatic hydrogen content. Although more than 40% of the spinning pitch obtained therein is quinoline-soluble mesophase, there is still remaining a large amount of quinoline-insoluble component, and therefore, spinning is conducted at a considerably high temperature.
It is easy to reduce the quinoline-insoluble component itself by, for example, employing a mild heat-treating condition. But, this leads to a significant decrease in the mesophase content and an increase in low-molecular-weight components which are soluble in a solvent such as xylene. This low-molecular-weight component which is soluble in xylene and the like will have an adverse effect to the orientation of the fiber while spinning, and evaporate at the spinning temperature giving a cause of the fiber cut off. Therefore, in order to prepare a spinning pitch with an excellent quality, it is not sufficient merely to decrease the content of exceedingly high-molecular-weight components which are insoluble in quinoline. Low-molecular-weight components which are soluble in xylene and the like must also be decreased, so as to make the pitch homogeneous and increase the content of intermediate components.
Various processes have been proposed other than those described above for preparing such homogeneous pitches. In one of the processes, an isotropic pitch is extracted by a solvent and the insoluble components are heat-treated at a temperature of 230.degree.-400.degree. C. (Japanese Patent Laid-open No. Sho 54(1979)-160427). Other processes comprise hydrogenation of an isotropic pitch in the presence of a hydrogen-donating solvent, followed by a heat treatment (Japanese Patent Laid-open Nos. Sho 58(1983)-214531 and Sho 58(1983)-196292). Still other process employs a repetition of a heat treatment on a pitch which was obtained by removing mesophase from a heat treated isotropic pitch (Japanese Patent Laid-open No. Sho 58(1983)-136835). Further, still other process can give a pitch containing 20-80% of mesophase by a heat treatment, and then recover the mesophase by precipitation (Japanese Patent Laid-open No. Sho 57(1982)-119984). The pitches prepared by these processes, however, are not necessarily satisfactory, i.e., some pitches have a sufficiently high mesophase content but not sufficiently low softening point, some have a sufficiently low softening point but do not have a sufficiently high mesophase content, some pitches have both a low softening point and a high mesophase content but contains a large amount of significantly high-molecular-weight mesophase which is insoluble in quinoline and the like and cannot be deemed as homogeneous pitch. None of these processes can provide pitches satisfying the following four requirements at the same time, that is: (1) a low softening point, (2 ) a high mesophase content, (3) a low quinoline-insoluble content, and (4) a low xylene-soluble contenent.
As a process for resolving these problems, Japanese Patent Laid-open No. Sho 61(1986)-138721 proposes a process for preparing a mesophase pitch comprising, subjecting a coal tar or heat-treated material of the same to a solvent extraction to obtain insoluble components, and hydrogenating and further heat-treating the insoluble components. The pitch produced by this process is a homogeneous pitch with a quinoline-insoluble content of below 20% and a mesophase content of above 90%. However, the strength of carbon fibers prepared from this pitch is not necessarily high enough according to the examples. The problem with this process resides in the fact that the solvent insoluble components existing in the starting material, coal tar, are not prepared for the purpose of producing spinning pitch for carbon fibers production. When solvent insoluble components which have originally existed in a raw material, coal tar or pitch, are separated and used as a spinning pitch, the properties of the spinning pitch or the characteristics of the carbon fibers are dependent upon the processes through which this raw material has been derived.
When a spinning pitch for carbon fibers production is prepared, not only the pitch must satisfy the aforementioned four characteristics by itself, but also it must produce carbon fibers with good characteristics.
Beside above-mentioned Japanese Patent Laid-open Nos. Sho 58(1983)-214531, Sho 58(1983)-196292, and Sho 61(1986)-138721, there are many processes proposed for effecting heat treatment after hydrogenation of a bituminous material such as pitches. These processes are effective in preparing spinning pitches with a low softening point. However, the most of these proposed processes take it for granted to use commercially available pitches or solvent-insoluble components contained therein, as they are, as a raw material for hydrogenation treatment. Since the raw material has not been specially prepared for the purpose of a spinning pitch production, the properties of the spinning pitch or the characteristics of the carbon fibers inevitably dependent upon the properties of the raw material. Therefore, there exists a desire for the development of a process which is capable of stably producing spinning pitches from which any possible factors of the fluctuation in raw material properties have been removed. The use of the process for increasing the yield of the solvent-insoluble components by the heat treatment of coal tar pitch involves the heat treatment of the solvent-ionsoluble components which have originally existed in the coal tar pitch, thus causing the formation of undesirable high-plymerized materials such as quinoline-insoluble components, and the like. If the solvent-insoluble components derived from such heat-treated materials containing the undesirable high-polymerized substances are used as a raw material for hydrogenation treatment, a great amount of solid materials must be filtered for separation after the solvent-insoluble components have been hydrogenated. This procedure of filtration and separation of the insoluble component contained in the hydrogenation solvent cannot always be performed effectively. There are various potential problems for scaling-up of this procedure, such as a slow speed of the filtration, clogging of the filter which makes it impossible to reuse the filter, and the like. Furthermore, if a raw material which may produce a large amount of insoluble component is used for this hydrogenation treatment, it is impossible to employ an efficient continuous process, such as the use of a tubular heater. Instead, the use of an inefficient batch-type treatment process is unavoidable.
A method of collecting solvent-insoluble components from coal tar pitch is described in the text of Japanese Patent Laid-open No. Sho 61(1986)-138721 in which it is stated that "Preferably, it can be performed using 5-20 times of solvent, at the boiling point or at a temperature near the boiling point, and for about 3-12 hr.". Thus the processes heretofore proposed are not necessrily efficient. Therefore, thorough consideration must be given also to the procedure for collecting insoluble components when a solvent-insoluble component is used as a raw material.
Accordingly, there is a desire for the development of a process for preparing a spinning pitch for the production of pitch-based high-performance carbon fibers, which satisfies the requirements for both the properties of the spinning mesophase pitch and the characteristics of the carbon fibers at the same time. Furthermore, the development of a process which is efficient and stable, and adapted to the scale-up, is desired.
We have already proposed processes for preparing pitches for the production of carbon fibers, i.e., Japanese Patent Laid-opens No. Sho 61(1986)-103989, No. Sho 61(1986)-238885 and No. Sho 62(1987)-277491. Although they are useful processes, they are still not sufficient enough to satisfy all of the requirements for the preparation of high-performance carbon fibers.
When the disclosures given in prior art are examined from another point of view, following facts can be recognized:
Examples of processes for preparation of pitches for use as raw materials for the production of carbon fibers are a process using a pitch obtained by hydrogenation or heat treatment of a specific type of polynuclear aromatic compounds (Japanese Patent Publication No. Sho 45(1970)-28013 and Japanese Patent Publication No. Sho 49(1974)-8634); a process comprising treating a petroleum-derived pitch-like material in the presence of a Lewis acid, followed by heat treatment (Japanese Patent Publication No. Sho 53(1978)-7533); a process comprising heat-treating a pitch with a specific range of aromatic hydrogen content (Japanese Patent Laid-open No. Sho 58(1983)-154792); a process comprising hydrogenating an isotropic pitch in the presence of a hydrogen-donating solvent, followed by heat treatment (Japanese Patent Laid-open No. Sho 58(1983)-214531) and Japanese Patent Laid-open No. Sho 58(1983)-196292); a process comprising heat-treating an isotropic pitch, separating and removing the produced mesophase, and heat-treating the pitch thus obtained (Japanese Patent Laid-open No. Sho 58(1983)-136835 and Japanese Patent Laid-open No. Sho 59(1984)-38280); and the like. The problem common to these processes is that the processes all utilize a batch-type heat treatment in their last step. As mentioned above, the production of spinning pitches requires the effective elimination of light fractions and a moderate degree of thermal polymerization. These must be done, however, under strictly controlled conditions in order to produce pitches which do not contain an unacceptable amount of infusible solid materials, and are free from light fractions which vaporize, or materials which decompose at the spinning temperature. The aforementioned heat treatment for the production of the spinning pitches is generally effected at a high temperature in the range of 350.degree.-500.degree. C. Carrying out this heat treatment using a batch system in an industrial-scale manufacturing facility involves difficulties in strictly controlling the operating temperature, pressure, time of treatment, and the like. Such difficulties increase as the amount to be treated per batch increases, and inadequate operations due to these difficulties tend to bring about undesirable results such as the formation of solid materials, insufficient elimination of light fractions, as well as fluctuation of the product properties between batches.
For these reasons, there has been a need for the development of continuous processes for preparing spinning pitches. One of the processes proposed comprises reducing and cracking a pitch-like material using a reducing solvent, and bringing the cracked material flowing down as a thin film into contact with an inert gas (Japanese Patent Laid-open No. Sho 59(1984)-88922). Another process proposes that a carbonaceous pitch be introduced into a thin-film evaporator, and be treated under specific conditions in the presence of an inert gas (Japanese Patent Laid-open No. Sho 60(1985)-238387). One feature common to these processes is to develop a thin film of pitches to enlarge their surface area so as to promote the rate of light fractions vaporization. Although these continuous processes may bring about better efficiency than those of batch processes, they present problems which remain to be resolved. When a pitch flows down by virture of its weight, for example, as in the process disclosed in Japanese Patent Laid-open No. Sho 59(1984)-88922, it does not form a uniform film if the flow amount is not sufficiently large. Rather, the pitch tends to flow down along a specific part of the wall (channeling), because the flow rate range, which allows the pitch film to develop evenly across the wall, is much limited. Thus, it is extremely difficult to develop a uniform pitch film. If the pitch to be treated in this process is a low-viscosity fluid, it is possible to develop what is known as the ideal piston flow. However, when the fluid has a high viscosity as in the case where spinning pitches are to be prepared, a uniform piston flow cannot always be developed. This produces fluctuations in the residence time of the material in the treatment zone, giving a wider residence time distribution. This wide residence time distribution, in turn, becomes the cause of fluctuations in the light fractions contents in the produced pitch, and also of fluctuations in the degree of thermal polymerization, which results in a heterogeneous pitch. Such heterogeneous pitches pose difficulties in the spinning operations required in the production of carbon fibers, and produces carbon fibers with extremely impaired properties, and which are thus unsuitable for use as raw materials for carbon fibers. In the process in which the downflow movement of a pitch relies only on its gravity, the residence time is dependent on the length of the wall in the vertical direction and on the viscosity of the pitch, making it difficult to control the residence time. Because of these reasons, Japanese Patent Laid-open No. Sho 59(1984)-88922 employs, in its examples, the means for providing a longer average residence time through the provision of a pump for circulating the pitch to be treated, and an overhead storage which permits the fluid to reside over a prolonged period of time. Since the pitch is continuously being taken out from the process as a product while being circulated, it is evident that the produced spinning pitch is a mixture of the pitches treated for different periods of time. Spinning pitches must be very homogeneous, as can be readily understood, because of the fact that they are to be spun into a fiber having a very small diameter of the micron order. Thus, a process which produces fluctuation in the treatment time is not preferable. The major reason for using batch systems in the heat treatment which is the last step in the above-mentioned various processes for the preparation of spinning pitches from heavy oils is to prevent this fluctuation in the treatment time.
In order to resolve this problem, the process of Japanese Patent Laid-open No. Sho 60(1985)-238387, which uses a thin-film evaporator, was proposed. This process produces a thin film of the pitch on the treatment vessel wall by mechanically forcing the pitch against the wall using a rotating blade. The thickness of the film can be controlled by changing the clearance between the blade and the vessel wall. This process, however, requires a large wall area onto which a thin film of the pitch is developed, which inevitably results in an increased production facility size as well as less economical production. More specifically, in order to shorten the treatment time for producing a specific amount of pitch exhibiting a desired quality, the pitch film must be as thin as possible to provide a larger area for evaporation. This leads to the need for a larger facility. If the thickness of the pitch film is increased to provide a longer residence time for the treatment, the rate of evaporation is retarded, and the light fractions can be eliminated only insufficiently. Thus, at all events, the process requires a large facility for achieving the satisfactory elimination of the light fractions. Since the Japanese Patent Laid-open No. Sho 60(1985)-238387 does not describe the details of the thin-film evaporator nor the amount of pitches to be treated, the details involved cannot be discussed here. It can be readily understood, however, that the process must involve feeding of a small amount of pitch to a facility with an unduly large vaporization area in order to provide an average residence time of 30 minutes at almost zero clearance, i.e., almost zero film thickness.
Using a higher temperature for the treatment is another way of shortening the treatment time. The use of a high temperature, however, gives rise to coking of the pitch on the wall and to formation of a solid film. The formation of cokes on the wall during a continuous operation can be the immediate cause of the changes in the clearance between the rotating blade and the wall, and thus changes the thickness of the pitch film. In the worst case, it interrupts the rotating movement of the blade. Raising the temperature to shorten the treatment time is, therefore, permitted only in a limited range. Another major problem in developing a pitch onto the wall of the vessel is the formation of cokes on the wall. If the film thickness changes from moment to moment during continuous operation due to coke formation, it is impossible to produce a homogeneous pitch over a long period of time. The problem is attributable to the situation specific to the preparation of pitches, especially spinning pitches for carbon fiber production which requires the heat treatment at a relatively high temperature in the range of 350.degree.-500.degree. C. for eliminating light fractions and effecting a moderate thermal polymerization, but requires, on the other hand, preventing the coke formation due to excessive thermal polymerization. This kind of situation does not exist in the handling of usual polymers. Simply elimnating solvents used and unreacted materials from such polymers merely requires a conventional thin-film evaporator, a current industry-wide practice without problems. The processes or facilities industrially utilized for eliminating light fractions at a relatively low temperature of below 350.degree. C. are not always effectively applied to the preparation of pitches.
As mentioned above, the preparation of pitches, especially spinning pitches for carbon fiber production, requires at the last stage the efficient elimination of light fractions, effecting a moderate thermal polymerization of the components, and depressing the coke formation. These three requirements have been met using the conventionally utilized continuous processes coupled only with the provision of a longer time for the treatment by circulating pitches or enlarging the treating facility. In view of this situation, a strong desire has remained for the development of an efficient and effective process for continuous treatment of pitches.